1. Field of the Invention
This invention is directed to the phase separation of an organic phase from an aqueous phase in the solvent extraction and/or stripping stages following the leaching of metal values from the ore containing that/those value(s) into an aqueous solution, and prior to the further refining of those values in processes, such as electrowinning, during mining operations for those metal values.
2. Background and Related Art
Most metals in elemental form are obtained from the ores in which they are found by first leaching the mined ores containing those metals as various compounds (“metal values”) in order to dissolve the respective metal values into an aqueous solution. The aqueous solution, now containing the metal values in dilute form, together with various impurities, such as iron and/or other minerals (known as the “pregnant leach solution” or “PLS”) may then, for example, be contacted, in the widely-practiced method of solvent extraction, by an organic solution containing an extractant reagent, which can complex with the desired metal values and pull them from the leach solution into a non-aqueous, organic phase. By using an acid of appropriate strength, the metal values may be subsequently stripped back out of the organic phase into another aqueous phase, which may be used as an electrolyte for an electrodeposition, or “electrowinning”, stage, wherein the elemental form of the metal may be captured.
In copper leaching operations, for example, sulfuric acid in an aqueous solution is contacted with a copper-containing ore, the acid is consumed and a/the copper content is introduced/increased in the aqueous solution. Copper in a dilute aqueous sulfuric acid solution is then commonly extracted in a solvent-extraction process by an bxime-based extractant in an organic solvent according to the chemical reaction:[2R−H]org+[Cu2++SO42−]aq ⇄[R2Cu]org+[2H++SO42−]aq  (1),where R−H is the oxime extractant. The resultant aqueous solution (also known as a “raffinate”), now depleted in copper and enriched in sulfuric acid, is returned to the leaching stage for further leaching of copper compounds from the mined ores.
By now contacting the copper-rich organic phase, in a “stripping” stage, with an aqueous solution (also referred to as a “lean electrolyte” or “LE”) having a high enough sulfuric acid concentration, the above chemical reaction, (1), may be reversed. The copper, which had been loaded onto the oxime reagent in the organic medium during the solvent-extraction stage, may be re-extracted into another aqueous medium (also referred to as a “rich electrolyte” or “RE”), which then has a relatively high concentration of copper and a lower level of sulfuric acid.
The rich electrolyte solution is then subjected to an electrowinning process in a “tankhouse”, where the rich electrolyte solution is passed through an electrolytic cell between an anode and a cathode. The electrical potential between the two electrodes causes copper to be deposited on the surface of the cathode as copper metal, and sulfuric acid is generated. The now copper-depleted aqueous solution (now the lean electrolyte) is typically recycled back to the stripping stage to again strip copper off the organic medium, and generally high-quality copper metal is removed from the cathode, with high electrical current efficiency.
A key factor in successfully operating a solvent extraction process is the ability to rapidly separate the aqueous and organic phases from one another in both the extraction and the stripping stages. In each stage, the organic and aqueous phases flow into a mixer where they are thoroughly and continuously mixed to form an emulsion—operating either in an “aqueous continuous” fashion, where the organic phase is distributed as fine droplets in the aqueous phase, or in an “organic continuous” fashion, where the water is distributed as fine droplets into the organic phase. The retention time in both mixing stages is typically one-to-three minutes, allowing sufficient time for the copper to be transferred between the respective phases. Following the mixing in both cases, the resulting emulsion then flows into a separating chamber, the settler tank—typically in a rectangular shape, where the residence time is typically five-to-eight minutes.
As the emulsion enters into the settler tank, the flow is broken up and distributed evenly across the tank width by means of one of a variety of “picket fence” styles, each style of which fits across the width of the tank, and has carefully-designed slots periodically along its length through which the emulsion can flow into the main tank area. Typically, the picket fence is designed in order that there are five-to-eight centimeters more of liquid level on the incoming side of the fence than on the outgoing side. Separation of the phases begins to occur almost immediately upon entering the settler, and as the phases proceed down the settler, the process of separation continues toward completion.
As phase separation takes place, one can see the formation of a clear organic phase, a dispersion or emulsion band, and a clear aqueous phase. As one travels down the length of the settler tanks, the width and depth of the dispersion or emulsion band shrinks and the size of the clear phases increases. Depending on the number, nature and amount of contaminants present in the organic and aqueous phases, as well as on the temperature and other factors, phase separation may not reach completion in either of the settler tanks. In these cases, the dispersion band remains quite thick throughout the settler tank(s), and, as the clear phases exit the tank(s), these phases may also carry along some of the emulsion from the dispersion band. If this occurs in the first extraction, or “E1”, stage, where the partially-loaded organic phase contacts the incoming PLS, a portion of the aqueous phase may be carried into the stripping stage, or if it occurs in the stripping stage, this results in the transfer of the impurities in the PLS into the electrolyte. It can also result in loss of organic diluent and extractant reagent in the exiting raffinate from a stage and in an increase in operating costs, for obvious reasons.
In a number of commercial operations, one solution to this problem of poor phase separation has been to add several kilograms of a clay, such as Filtrol F1 (available from Engelhard), to the mixer of either/both the solvent extraction and/or the stripping stage(s). Unfortunately, while effective in causing the collapse of the dispersion band and in improving phase separation, the addition of clay results in the formation of crud, a troublesome, solid, stabilized emulsion. Typically, crud is formed, to some extent, in all circuits, but formation of additional crud in either, much less both, the solvent extraction and the stripping stages is undesirable.
Crud takes up space in the settler tank, making the settler less efficient in terms of phase separation. Crud must be periodically removed at a cost in manpower, and its removal also results in loss of expensive extractant and organic diluent. In addition, some solids may be transferred into the electrolyte, causing irregular copper deposits on the cathodes, thus increasing the number of cathodes rejected for quality considerations and/or reducing the value for which the cathodes may be sold.
We have now surprisingly found that the use of clay may be effectively replaced by the addition of crystals of a metal salt (e.g., copper sulfate in a copper extraction process, nickel sulfate or nickel ammonium sulfate in a nickel solvent extraction system, and so forth) to the mixture of the aqueous and organic phases in either or both of the mixers in the solvent extraction and/or stripping stages. The addition of the metal salt crystals results in an effective collapse of the dispersion band, and may also promote collection of contaminants from the organic phase at its interface with the emulsion band, making those contaminants easier to remove. The metal salt crystals dissolve in the aqueous phase and do not contribute to crud formation, as the clay does, and the added metal (i.e., copper, nickel, etc.) ions are not lost, but may be recovered in the solvent extraction process.
The copper sulfate crystals may be readily obtained by taking a small portion of the incoming PLS and processing it through a small solvent extraction plant designed to recover copper sulfate crystals from the strip solution. This is a technology that is practiced commercially in a number of small plants.